Lysosomal storage diseases (LSDs) are single gene defects that result in the deficiency of a catabolic lysosomal enzyme and accumulation of one or more of its macromolecular substrates in the lysosome. GM1 gangliosidosis results from deficiency of the lysosomal enzyme [unreadable]-galactosidase ([unreadable]gal), leading to progressive and fatal neurodegeneration. Human GM1 gangliosidosis occurs as three types based on disease severity and age of onset, classified as infantile, juvenile, and adult-onset forms. Gene therapy with adeno-associated viral vectors is a very promising strategy for treatment of GM1 gangliosidosis based on extraordinary preliminary results in the knockout mouse model. However, the mouse brain is 1,000-2,000 times smaller and much less complex than the brain of a human infant, requiring that results obtained in mice be reproduced in an animal with a brain size and complexity more similar to humans. The well-characterized feline model of GM1 gangliosidosis, with a brain size only 15 times smaller than a human infant, will be utilized to test therapeutic efficacy of AAV vectors in a larger and more complex brain. In addition, preliminary mouse studies employed a mouse [unreadable]-galactosidase cDNA, although AAV vectors for human clinical trials will express human [unreadable]gal. Therefore, it is necessary to perform bio-equivalency studies to ensure the functionality of the human [unreadable]gal cDNA before initiating human clinical trials. The experiments in this application are designed to advance AAV gene therapy toward human clinical trials through the following specific aims. In Aim 1, short-term therapeutic studies will be performed after intraparenchymal or CSF-mediated delivery of AAV vectors to the feline GM1 brain. Also, a direct comparison of human and mouse [unreadable]gal will be conducted in short-term experiments in GM1 mice. In Aim 2 long-term studies of therapeutic efficacy in GM1 cats and mice will be conducted using clinical, behavioral, and biochemical assays of disease progression. Finally, Aim 3 will test the principle of in vivo selection of an AAV capsid library (molecular evolution) to identify AAV vectors with tropism for the GM1 mouse central nervous system (CNS) after intravascular (iv) infusion, and then test their therapeutic efficacy in these mice. Validation of this approach to generate AAV vectors with CNS-tropism after iv infusion will pave the way for development of such vectors for human application. PROJECT NARRATIVE: The AAV vector-based approaches investigated in this project for CNS therapy in GM1-gangliosidosis are likely to be applicable to many, if not all neurodegenerative lysosomal storage diseases. The experiments proposed here are essential to determine the scalability and therapeutic efficacy of each approach applied to a much larger and complex brain such as that in GM1-gangliosidosis cats. Moreover the AAV vectors encoding the human lysosomal [unreadable]-galactosidase enzyme developed and tested here for their therapeutic efficacy in GM1 mice and cats may ultimately progress into human clinical trials for this devastating disease for which there is no treatment. Finally, the investigators are proposing to develop a new generation of AAV vectors capable of targeting the brain through the vasculature. This new generation of AAV vectors may provide the means to develop highly effective gene therapy approaches to treat many childhood, adult, as well as geriatric neurological diseases currently beyond the reach of modern medicine.